docker: init fetchdocker nix code for docker2nix

This change adds granular, non-docker daemon docker image fetchers and
a docker image layer compositor to be used in conjunction with the
`docker2nix` utility provided by the `haskellPackages.hocker` package.

This change includes a hackage package version bump and updated sha256
for recent fixes released to `hocker` resulting from formulating this
patch.

pkgs/build-support/fetchdocker/fetchDockerConfig.nix

@@ -0,0 +1,13 @@
+pkgargs@{ stdenv, lib, haskellPackages, writeText, gawk }:
+let
+  generic-fetcher =
+    import ./generic-fetcher.nix pkgargs;
+in
+
+args@{ repository ? "library", imageName, tag, ... }:
+
+generic-fetcher ({
+  fetcher = "hocker-config";
+  name    = "${repository}_${imageName}_${tag}-config.json";
+  tag     = "unused";
+} // args)